Cooper is 2 years old and has kidney degeneration/ failure, urine analysis shows protein levels as through the roof, pH is normal, SG is normal (with the refractometer). Ive i was given the option of blood tests but opted against it as i dont really want to put him through being poked and proded when i feels its already fairly advanced and he probably hasnt got particularly long left.

HLD has been present for the last 6+ weeks, which has gotten significantly worse over the last few weeks. Significant weight loss, and sleeps alot but hes still happy, begs to come out of the cage, climbs the bars a little bit for treats and is very happy spending free range time snuggled up with me.

The vet did give him a week of antibiotics as his leukocytes were high, but that hasnt made any difference.

Also rat guide says "The recommended dosage is based on 1 to 5 mcg’s (micrograms) of B12 in the B complex, and is given PO (oral) daily" and "dissolving the powder in 2mL of water".... "the dosage may only be about 0.2mL".

if my B- complex supplement contains 1ug of B12 then he should be having 1/10th of a tablet right?

I keep thinking i understand, then start doubting myself because the rat fan club page says "B12 50 mcg/kg of diet 0.66 - 0.96 mcg per rat per day"

A growing rat requires about 0.5mg iron per day (based on requirements of 35mg iron per kg diet and assumed food intake of 15g per rat per day), so use that as a guideline I guess. A fully grown rat probably needs less, and there'll be iron in his food to begin with anyway. Maybe give him enough Spatone for 0.25mg iron per day? I don't know offhand how excess iron could affect an elderly animal, but I do know that rather large doses of iron in humans are generally okay short-term at least (it's the standard treatment for iron deficiency anaemia) and any excess just goes down the toilet. And turns your poo an alarming colour.

The B12 daily requirement for a growing rat is about 0.75μg (based on requirements of 50μg/kg diet and assumed food intake of 15g per rat per day). So maybe half a tablet daily, since a fully grown rat probably needs less than a growing one. This does actually match with the Rat Guide guidelines: "may only be about" suggests that 0.2mL is the low end of what they expect to dispense, so that would be based on having the top end of the capsules i.e. 5μg per capsule, which works out to 0.5μg per dose.

The Rat Fan Club seems to be basing its advice on Nutrient Requirements of the Laboratory Rat, same as me, but assuming higher food intake. Whereas I assume 15g per rat per day, because:

If cited papers provided nutrient intakes per day but did not specify dietary concentrations, the values have been converted to dietary content by assuming a dietary intake of 15 g/rat/day for growing rats or adult rats at maintenance.